Since an Ag alloy film in which Ag is the main component of a certain thickness or larger has a high reflectance in visible light and a low electrical resistance, it is expected to applicable to a reflecting electrode of a liquid crystal display device, an organic EL display device, or the like.
An Ag alloy film, however, does not form a passivation film and is susceptible to external effects. Specifically, an Ag alloy reacts with sulfur to form silver sulfide and reacts with halogen to form silver halide. An Ag alloy is also liable to be agglomerated by being subjected to heating.
The Ag alloy film thus has a problem of degradation in terms of the high reflectance and the low electrical resistance, which are inherent characteristics of an Ag alloy film, upon being subjected to a thermal hysteresis in the course of manufacturing process of the display device or the like. Novel Ag films having those different from conventional alloying compositions have been proposed by taking the problem regarding the Ag alloy film into consideration.
Patent Document 1, for example, discloses an Ag alloy film comprising one or two kinds of element selected from a group consisting of Bi and Sb in a total amount of 0.01 to 4 atomic %, which has a high reflectance inherent in Ag and circumvents the degradation in terms of reflectance with time by suppressing agglomeration and crystal grain growth.
Patent Document 2 discloses an Ag-based alloy film constituting a reflecting anode electrode in an organic EL display device. It is demonstrated by adding 0.01 to 1.5 atomic % of Nd or 0.01 to 4 atomic % of Bi to the Ag-based alloy film that the dark spot phenomenon in an organic EL display device can be successfully circumvented by exertion of the effect of the elements to prevent agglomeration of Ag.
Patent Document 3 discloses that a high reflectance can be achieved by adding Bi to Ag to suppress crystal grain growth and agglomeration in an Ag film as well as by further adding V, Ge, and Zn within a range which satisfies a predetermined expression.
Patent Document 4 discloses that an Ag-based alloy having heat resistance and corrosion resistance may be attained by containing a predetermined small amount of Cu and Te/Se, and further adding In, Sn, Zn, Pd, Au, Pt, Ru, Ir, Fe, Ni, Bi, and P as needed. Patent Document 5 indicates an improvement in terms of heat resistance in an Ag alloy film obtained by using a target which is prepared by adding a specific small amount of Bi along with In, Sn, Zn, Au, Pd, and Pt.
The Ag film is generally subjected to a cleaning treatment such as an UV irradiation or an O2 plasma treatment after deposition in the manufacturing process of the display panel. The treatment, however, causes a problem of oxidation and blackening Ag. The blackening is induced by chemical reaction of Ag with highly reactive oxygen radicals produced in the course of the UV or O2 plasma irradiation.
In a top-emission type OLED display panel in which the light is emitted from the opposite side of the substrate, an organic material layer is laminated on top of a reflecting electrode consisting of a single layer Ag alloy film or a reflecting electrode comprising an Ag alloy film. For the purpose of securing electrical connection of the reflecting electrode with the organic material, the surface of the reflecting electrode is always cleaned by being subjected to the treatment such as an UV irradiation or an O2 plasma treatment prior to the deposition of the organic material in the course of the manufacturing process of the display panel.
The Ag film, however, has a problem of forming silver oxide and being blackened upon being subjected to an UV irradiation or an O2 plasma treatment as described above. The formed silver oxide causes a short-circuited device element. When the resistance to oxidation is not high, production yield of the device element is deteriorated.
The formation of a silver oxide is attributed to the fact that Ag is liable to be oxidized by active oxygen generated by an UV irradiation or an O2 plasma treatment because Ag does not form a passivation as described above.
In order to suppress the deterioration, specifically the blackening of Ag alloy films by the oxidation, upon being subjected to the cleaning treatment, a means to protect the Ag film is employed by depositing either a transparent conductive oxide film such as ITO or an oxide film either directly on or directly beneath the Ag film. Even if the ITO or the like is laminated on and beneath an Ag alloy film in such a manner, there is a case in which the Ag film is incompletely protected and deteriorated upon being subjected to the cleaning treatment due to causes such as non-uniformity in thickness or presence of pinholes in the ITO film or the like. Then silver oxide is formed and the reflectance is deteriorated. It is thus required for the Ag alloy film itself to have excellent resistance to the cleaning treatment such as an UV irradiation and an O2 plasma treatment, that is, resistance to active oxygen (occasionally referred to as resistance to oxidation hereinbelow).